1. Field of the Invention
This invention relates to gas-liquid mixing operations. More particularly, it relates to batch gas-liquid mixing wherein a substantial variation in liquid volume occurs over the course of the mixing operation.
2. Description of the Prior Art
An advantageous gas-liquid mixing process and apparatus suitable for batch processing operations is disclosed in the Litz patent, U.S. Pat. No. 4,454,077, and the related reissue patent, U.S. Pat. No. Re. 32,562. This gas-liquid mixing technology has been referred to in the art as the Advanced Gas Reactor (AGR) system. As commonly and advantageously employed for a variety of practical commercial applications, the AGR system incorporates an open-ended hollow draft tube in a gas-liquid mixing vessel adapted to contain a body of said liquid. A down-pumping impeller positioned within the hollow draft tube is employed to pump liquid in said body of liquid into the top of said hollow draft tube for discharge at the bottom thereof and overall movement in a recirculating flow path in the mixing vessel. Because of such recirculation of the liquid downward in the hollow draft tube and upward in the vessel outside said tube, and aided by the positioning of baffles at the top of said draft tube, vortices are formed in the inlet area of the draft tube, such as to draw feed gas from the overhead space above the liquid level in the vessel into the recirculating liquid passing downward into the draft tube. It is this gas ingestion mechanism that provides a major benefit of the AGR system.
Satisfactory vortex development for such gas-liquid mixing purposes depends, among various practical operating factors, on the maintaining of a proper liquid level above the top of the draft tube. At any particular liquid flow rate down the draft tube, the maximum gas ingestion will occur at a particular operating liquid level in the vessel above the draft tube. The liquid flow rate, in turn, is a function of the impeller-draft tube design and the rotational speed of the impeller. Thus, for a 9" double helical impeller, with a single 9" pitch length, running at 400 RPM, in water, within a baffled 10" inside diameter draft tube, the optimum liquid level is about 4" above the top of the draft tube. If the liquid level were about 8" above the top of the draft tube, however, the vortices would typically no longer form, and the gas ingestion rate would drop essentially to zero. Thus, operating at liquid levels above the optimum level can substantially reduce the gas ingestion capabilities of the AGR system.
In gas-liquid mixing operations subject to such non-optimum liquid levels, therefore, the effectiveness of the AGR system in achieving enhanced gas-liquid mixing may be seriously impaired. Typical gas-liquid mixing applications in which an increasing liquid level may cause such non-optimum operation of an AGR system involves the production of aluminum alkyls or the hydrogenation of nitro compounds. When nitrobenzene is hydrogenated to form aniline, for example, water is formed as a by-product. If the AGR reactor (mixing) vessel were initially filled to the optimum level of liquid nitrobenzene to obtain good gas ingestion, via the vortex mechanism referred to above, the level of liquid would rise as the reaction proceeded. Thus, the by-product water produced would increase the liquid volume and cause the liquid level to rise in the vessel. A point would be reached when the liquid level became so high that the vortices would no longer form. At this stage of the operation, the reaction would stop because of the lack of hydrogen gas bubbles in the liquid phase. It will also be appreciated that, prior to reaching such stage, the rise in liquid level can be such that vortex formation and gas ingestion are less favorable than occurs when the liquid level is at the optimum level for the particular system.
In large commercial units for the carrying out of such reactions, the liquid level might desirably be, for example, about 18 inches, plus or minus 2 inches, above the draft tube. In such AGR applications, a volume change from the beginning of a batch to the end thereof may be on the order of 20 to 150% of the starting volume in the mixing vessel. In typical AGR reactors employed for such process reactions, this volume change will result in a liquid level change ranging from about 12 to 60 inches. As indicated above, such liquid level variations can cause a significant decrease in the effectiveness of the AGR system, even to the point where the desirable AGR system cannot be employed for gas-liquid mixing operations of practical commercial significance.
Because of the highly desirable gas-liquid mixing action achievable by the use of the AGR system, there is a desire in the art for further developments enabling the AGR system to be operated, and its benefits achieved, over a broader range of liquid levels. Such developments, enabling the AGR system to accommodate applications subject to a non-optimum liquid level, as for example, a rising level of liquid within the mixing vessel, would enable the use of the AGR system to be desirably extended to a wider variety of practical gas-liquid mixing operations for which enhanced mixing, and enhanced gas utilization, are desired in commercial operations.
It is an object of the invention, therefore, to provide an improved AGR system and process for gas-liquid mixing.
It is another object of the invention to provide an AGR system and process capable of effective operation over a substantial range of liquid levels in the course of a given gas-liquid mixing operation.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.